Compressor Basics
An air conditioner and heat pump can be considered big refrigerators and use the process of refrigeration to maintain desirable conditions. HVAC Compressors make up the heart of most refrigeration systems. Of course every refrigeration systems needs all of its working components to function properly but one of the key components of a refrigeration system is the compressor. While the compressor pumps the refrigerant throughout the closed loop refrigeration system, it is doing more than just pumping refrigerant. It compresses the vapor refrigerant. Under the compression process the temperature of the vapor rises. One of the key purposes for raising the temperature of the refrigerant is make its temperature warmer than the outside ambient temperature where the condenser is located. When the cooler outside ambient air temperature, the HVAC condenser fan motor forces this cooler air through the condenser coils. The HVAC refrigerant or freon as many refer to it, is inside these coils and loses or rejects heat into the cooler ambient air. It is a heat exchange process where heat leaves the refrigerant and is forced into the outside air. The HVAC refrigerant cools off enough in the condenser to condense or change state. The HVAC refrigerant changes state from a gas or vapor to a liquid. The HVAC compressor facilitates this by pumping the refrigerant and raising the temperature of the vapor above outside air ambient temperature. Without the HVAC compressor, many air conditioners or heat pumps would not be able to function at all. *Not all HVAC refrigeration systems need a compressor to facilitate what is mentioned in this article. There are absorption and ammonia systems which use natural gas to heat the refrigerant rather than a compressor.
Compressor Types
Reciprocating Compressor The reciprocating compressor is the most common HVAC compressor in use today. These can be hermetically sealed compressors or semi-hermetically sealed compressors. Hermetically sealed compressors are more common in residential and light commercial uses and these compressors can be found in everything from the smallest refrigerators, freezers, and ice machines to large air conditioning condensing units. Semi-hermetic compressors are typically found in larger commercial HVAC systems for refrigeration and air conditioning applications.
A reciprocating compressor has piston(s), valves for intake and discharge, and an electric motor which drives piston(s) for compression. Larger compressors, such as the one pictured, has an unloader or unloaders (some systems have loaders versus unloaders) on it for the purpose of running at partial capacity. This is an energy efficiency feature which allows the compressor to use less energy under low demand situations. All compressors use some sort of oil. The type of oil depends on the type of refrigerant used and it is important to maintain the proper level of oil in the compressor. Some HVAC compressors, especially larger compressors, are equipped with low oil cutoffs which will prevent the compressor from running if a problem exists with the oil level.
Scroll Compressor
While the recirpocating compressor is one of the most common compressors seen in HVACR equipment today the reciprocating compressor is slowly losing its position to the scroll compressor. Scrolls began appearing in the eighties and slowly proved it was worthy of its place in HVACR equipment proving it was a good alternative and reliable. Efficiency of the scroll compressor to the reciprocating compressor can be debated especially when you compare volumetric efficiency compared to mechanical efficiency. Clearly, the scroll compressor is superioir in volumetric efficiency and the reciprocating is superior in mechanical efficiency as proven in this white paper document courtesy of Bristol Compressor. What is fact about the scroll compressor and what makes it more popular among manufacturers of HVACR equipment is it has less moving parts. This feature gives the scroll compressor an advantage over other compressors and allows the HVACR manufacturers to offer longer equipment warranties. The air conditioner scroll compressor is clearly overtaking the reciprocating compressor in the HVACR market today. The air conditioner scroll compressor, also found in heat pumps, offers efficiency and reliability and that is why the air conditioner scroll compressor is winning in the market place today.
Scroll Two-Stage Compressor
Copeland, the company that manufactures the air conditioner scroll compressor, offers a two stage scroll compressor that allows increased efficiency of HVACR equipment. It does this by shifting a mechanical plate inside the scroll compressor. The plate has two holes in it. One hole is for 100% operation and the other hole is for 65% operation. In low demand situations the plates shifts to the 65% hole allowing only 65% of the refrigerant through the hole and into the system. In high demand situations the plate shifts to the 100% hole allowing 100% operation. This increases efficiency by decreasing the load on the electric motor inside the compressor. The electric motor in the compressor draws a lot of electricity. By decreasing the load on the air conditioner scroll compressor you decrease the electricity the compressor uses. This saves money in energy costs. Rotary Compressors Rotary compressors are most commonly used in small tonnage applications. These small reliable compressor are quiet and generally used in applications under three tons. This tonnage capacity of three tons makes the rotary compressor ideal for use in window units, mini-splits, and small air conditioning and heat pump split systems. While the rotary air conditioning compressor is not as efficient as the scroll or reciprocating compressors the offer a higher reliability because of less moving parts. The other benefit that separates the rotary compressor from the scroll and the reciprocating compressors is that the rotary is quieter that the scroll or reciprocating compressor.
Screw Compressors
Screw Compressors are generally used in chiller systems that have large capacities. The screw compressor uses high-pressure refrigerants and can most often be found serving chilled water systems in large commercial building or in some industrial cooling applications. The major benefit for the screw is that they offer variable or modulating capacity control. Unloaders are unnecessary as a variable frequency drive can speed up or slow down the screw compressor based on demand
capacities. This allows the screw compressor to use only the energy necessary to keep up with demand. This same modulating compressor technology is being adapted to smaller refrigeration, air conditioning, and heat pump systems that use scroll and reciprocating compressors for the compression process in cooling (and heating for heat pumps) applications. The energy savings using a modulating type of compressor for cooling applications is greater than other systems available. Centrifugal Compressors
Centrifugal compressors are used in very large cooling applications for chilled water systems. These compressors run at very high R.P.M.'s and generally require a separate motor for pumping oil to the bearings and moving parts in the centrifugal compressor. The centrifugal compressor uses centrifugal force via an impeller to create a pressure differential from the low side of the refrigeration circuit to the high side of the refrigeration circuit.
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